This invention relates to a system for monitoring tire pressure in wheeled vehicles. More specifically, the invention is directed to a remote tire pressure monitoring system that monitors the tire pressure for each tire and indicates to a vehicle operator the unique location of each tire being monitored.
It is known to equip vehicle tires with mechanisms to indicate tire pressure. For example, it has been taught to include tire pressure monitors that sense the air pressure in a tire and to transmit that tire pressure information to a vehicle operator via electronic circuits and radio transmitters. The tire pressure monitors are mounted within the tire air valve stem, and they transmit pressure readings at predetermined time intervals using radio frequency signals directly to a centralized tire pressure monitoring receiver. However, such known tire pressure monitoring systems lack a mechanism for reporting pressure information for specific tires. Accordingly, when the tire pressure monitors communicate that tire pressure is low, the vehicle operator has to manually inspect and determine which specific vehicle tire or tires have low pressure (for example, the right front tire or the spare tire, etc.)
To aid in specific vehicle tire location identification, other tire pressure monitoring systems have been employed that include tire pressure monitors that transmit unique identification codes to a central controller of the vehicle. Using the unique identification code, the central control determines from which of the tires the tire pressure signal was sent. However, there are drawbacks associated with such systems. One such drawback is that the central controller must be re-programmed each time the tires are rotated or otherwise moved from their original locations. This is because the central controller can use the unique identification codes sent by the tire pressure monitors to identify the correct tire location only if the central controller knows in which location the tire is currently mounted. When the tires (and thus the tire pressure monitors) are moved from their original locations on the vehicle, the service person must re-program the central controller so that it will be able to associate a particular monitor code with the correct tire location on the vehicle. This situation is undesirable because proper operation of the tire pressure monitoring system then depends upon periodic human intervention, which introduces a possible area for error or undependability.
To solve this problem, the inventor hereof has previously invented a new tire pressure monitoring and location identification system that is the subject of co-pending U.S. patent application No. Ser. 09/607,302. In this previous patent application, the inventor describes a system wherein each vehicle tire is equipped with a pressure monitor that is capable of sensing and transmitting tire pressure information via radio frequency signals (or inductive, magnetic, ultrasonic, or infra-red signals). For each tire pressure monitor, a corresponding transponder capable of receiving radio frequency (RF) signals is fixedly attached to the vehicle in a proximate location to the corresponding pressure monitor, such as in the corresponding wheel-well. According to this previous invention, each of the transponders has a limited signal reception range so that each transponder only receives tire pressure information from the corresponding pressure monitor. Each of the five transponders (one for each tire, including the spare) includes a unique pre-programmed identification code. When a transponder receives pressure information from its corresponding pressure monitor, the transponder transmits a data packet, which includes both the tire pressure information as well as the corresponding transponder""s unique identification code, to a central system receiver in the vehicle. Because all of the transponders are fixedly attached to the vehicle, the central system receiver is able to identify which tire pressure information corresponds to which tire from the transponder identification code.
However, the inventor hereof has now discovered that the transponders sometimes receive stray signals from sources other than their corresponding tire pressure monitors. For example, depending upon the positioning of the vehicle and other factors, the transponder corresponding to the right front tire of the vehicle may receive tire pressure signals from any of the other four tire pressure monitors (including the spare) in addition to the right front tire pressure monitor. Further, if two vehicles having tire pressure monitors are parked near each other, it is possible that a transponder of one of the vehicles may receive signals from pressure monitors installed on the other vehicle. Accordingly, sometimes a transponder may transmit tire pressure information to the vehicle""s central system receiver that is not indicative of its corresponding tire.
Thus, there is a need for a further refined tire pressure monitoring system that more accurately and consistently reports tire pressure information corresponding to a particular tire.
The present invention is directed to a tire pressure monitoring and identification system. The system includes a plurality of tire pressure monitors that transmit tire pressure information that may be received by a plurality of transponders. Each of the transponders is fixedly attached to the vehicle proximate to its respective corresponding tire pressure monitor, such as in the corresponding wheel-well. Each transponder has a unique identification code. When a transponder receives a signal indicative of tire pressure information, it also detects the strength of the received signal and derives a Received Signal Strength Indication (RSSI) value therefrom. The RSSI signal is typically a DC voltage that is indicative of the strength of the tire pressure signal received by the transponder. A tire pressure signal received from a corresponding tire pressure monitor (which is positioned proximate to the receiving transporter) will generally be significantly stronger than a tire pressure signal received from a non-corresponding tire pressure monitor (which is positioned relatively further away). According to the present invention, the system determines if a tire pressure signal received by a particular transponder was actually transmitted by the corresponding tire pressure monitor (as opposed to being a stray signal from a non-corresponding pressure monitor) based upon the comparative strength of the received signal, as measured by the relative magnitude of the RSSI value.
The relative strength of a tire pressure signal received by a particular transponder can be used in a variety of ways to determine if the signal was transmitted by the corresponding tire pressure monitor. According to a first preferred embodiment of the invention, a transponder that receives a tire pressure signal compares the associated RSSI value to a reference value. If the RSSI value is greater than the reference value, then it is determined that the tire pressure signal was transmitted by the corresponding tire pressure monitor, and the transponder transmits the tire pressure signal to a central system receiver. If, on the other hand, the RSSI signal is less than the reference value, then it is determined that the signal received by the transponder was a stray signal from a non-corresponding pressure monitor, and the transponder does not transmit it to the central system receiver. Thus, in the case where a tire pressure monitor transmits a tire pressure signal that is received by multiple transponders, only the corresponding transponder will transmit the signal on to the central system receiver.
According to a second preferred embodiment of the invention, the magnitude of the RSSI signals are compared to each other by a central system controller. In particular, when a tire pressure monitor transmits a tire pressure signal that is received by multiple transponders, all of the transponders then transmit the tire pressure signal, the RSSI signal and the transponder""s unique identification code to the central system receiver. The central system receiver provides the data signals to the central system controller, which compares the various RSSI signals to each other. The central system controller determines that the transponder associated with the highest RSSI signal corresponds to the tire pressure monitor that transmitted the tire pressure signal. Accordingly, the central system controller indicates that the received tire pressure information corresponds to the tire associated with the transponder providing the highest RSSI value.